HMX is a crystalline solid used as an explosive and as an ingredient of propellants, primarily in military applications. HMX is crystalline octahydro-1,3,5,7-tetranitro -1,3,5,7-tetrazine, alternatively known as cyclotetramethylenetetranitramine; 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane; homocyclonite; octagen; or by Chemical Abstracts Registry No. 2691-41-0. It is polymorphic, having four crystalline forms commonly known as alpha HMX, beta HMX, gamma HMX, and delta HMX. Beta HMX is considered to be the least impact sensitive of the four, and for that reason is the only form of HMX which meets present military specifications. For reasons of safety and efficiency, it is desirable to crystallize HMX in the beta form exclusively.
Recently, a process has been disclosed to obtain HMX in the beta form substantially exclusively, namely in U.S. Pat. No. 4,785,094 of the present Applicant, which issued Nov. 15, 1988 and is assigned to Morton Thiokol, Inc. In said process an unsaturated solution of HMX in a solvent consisting essentially of nitric acid is added to a dispersion of beta HMX seed crystals in water to precipitate beta HMX selectively from the solution of HMX.
This aforementioned precipitation step for the production of beta HMX crystals is the last stage of a process beginning with the reaction of hexamine and acetic anhydride. The reaction sequence for the production of the unsaturated HMX solution to add to the dispersion of beta HMX crystals in water is as follows:
(a) production of diacetyl pentamethylenetetraamine (DAPT): ##STR1## (b) reforming hexamine by using ammonia to react with the formaldehyde generated in the DAPT production: EQU 6 CH.sub.2 O+4 NH.sub.3.fwdarw.hexamine+ 6 H.sub.2 O PA0 (c) production of 1,5-diacetyl-3,7-dimitro-1,3,5,7-tetraazacyclooctane (DADN): ##STR2## with the formaldehyde generated reacting with urea to form dimethylol urea (DMU) EQU 2 CH.sub.2 O+urea.fwdarw.DMU PA0 to prevent the formaldehyde from reacting with nitric acid PA0 (d) production of unsaturated HMX solution: ##STR3##
An important economic consideration in this improved process for the production of beta HMX resides in the ability to recover the sulfuric and nitric from the process steps. In the recovery of acid from the process, an important consideration would be the ability to destroy or isolate any organics (mostly DMU, HMX and DADN fines) in the spent acid stream.
Owing to the physical behavior of water/nitric acid systems, it is not possible by a standard distillation or rectification process to remove excessive water from the acid since the mixture forms an azeotrope with a maximum boiling point at about 69% wt HNO.sub.3 at 120.5.degree. C. Thus, it has been known for a long time that by adding sulfuric acid to a nitric acid/water mixture the boiling behavior of the latter can be altered so that concentrated nitric acid can be distilled off and concentrated sulfuric acid recovered in a sulfuric acid concentrator. Such a process is reviewed, for example, in Gericke, D.; Concentration of Nitric Acid by Sulphuric Acid, Chemie-Ingenieur-Technik 21/74 pp. 894-899.
Thus, it appeared that an easy and convenient solution to the recovery of acids in the hereinbefore mentioned improved process for the production of beta HMX would reside in mixing the spent acid stream so that strong sulfuric acid could break the nitric acid/water azeotrope to permit stripping of substantially all the nitric acid out of the mixture at a level of about 98% minimum concentrated nitric acid. Thereafter, the remaining sulfuric acid and water mixture could be sent to a sulfuric acid concentrator to boil off the water and recover about 93% minimum concentrated sulfuric acid. It was also to be assumed that the combination of hot sulfuric and nitric acid would lead to the destruction of any organics, such as DMU, DADN and/or HMX in the spent acid stream. However, it was discovered that such an apparent solution to the recovery of spent acids in the improved beta HMX production process presented various undesirable drawbacks, such as for example, high capital and operating costs, the non-destruction of DMU in the hot, spent mixed acid stream, the non-flexibility of the feed stream concentrations and the mixing or integration of the nonexplosive portion of the process (DAPT/DADN production) with the potentially explosive portion of the process (HMX production and recrystallization).
Thus, a need arose for an acid recovery system for the beta HMX production process that would avoid most of all of these drawbacks and which would be economical and efficient to operate.